Prior art solar engines usually use an external heat exchanger or receiver to convert the sunlight to heat. Systems that heat fluids externally fail to utilize all the energy incident on the receiver because of losses in the heat collection, transport, and delivery systems. The transfer of energy in such systems is dependent on the mass flow of the working fluid, and is therefore limited by fluid flow through pipes, valves, and heat exchangers. Another consequence of externally heated systems is that radiant to thermal converters usually operate at significantly higher temperature than the working fluid, resulting in inncreased radiation losses and larger probability of failure due to thermal stresses and cycling. Engines that operate with externally produced heat that have been used with sunlight as the power source can include those using the Rankine, Brayton, and Stirling cycles.
The Stirling engine is a closed cycle, reciprocating engine that utilizes the energy in an externally heated working fluid. In solar powered Stirling engine designs, concentrated sunlight is usually directed onto a metal or ceramic a receiver that then transfers the heat by conduction to the working gas. The heated gas expands against a piston doing mechanical work. In addition to the working piston there is a transfer or displacer piston that moves the gas to and from the heating chamber. The expanded gas is passed through a recuperator to recover heat for the next charge. The use of concentrated sunlight to operate a Stirling cycle engine with a heat exchanger with a common surface radiantly heated and in contact with the working fluid was discussed by Daniels in his book "The Direct Use of the Sun's Energy" (Ballantine Books, 1974) and a working model was described in 1962 by Finkelstein in the 1961 meeting of the American Association of Mechanical Engineers (Paper 61-WA-297). These engines use a transparent window to allow the sunlight to enter the cylinder continuously.
There are several complicating factors that must be overcome to achieve a viable solar Stirling engine. These include: (1) the heat exchanger to transfer the solar energy to the gas, (2) the relative motion of the displacer piston and the working piston, (in free piston designs), (3) the sealing of the gas charge, and (4) the recuperation of heat from the expanded gas to supply it to the next charge, and (5) an external cooling system to remove the waste heat from the sealed gas charge.
A better means of introducing the heat to the engine is to use the radiant energy to heat the working fluid directly within the engine, thus eliminating problems encountered with conventional radiant heat exchangers. U.S. Pat. No. 4,313,304 discloses a radiant energy collection and conversion apparatus and method that utilizes a suspension of particles to directly absorb the concentrated sunlight and heat a working fluid and vaporize the particles after they have provided the heat exchange.
In another prior device that is disclosed in U.S. Pat. No. 4,089,174 issued on May 16, 1978 to Mario Posnansky, a cycle heat engine includes a window permeable to sunrays and a rotary expansion chamber containing a heat transfer medium. There are several problems with such a system; the closed cycle operation requires heat exchangers to remove the waste heat, a heat absorbing fluid with good solar absorption characteristics without degradation in the closed must be found, and the motion of the sliding seals against the window can cause damage to the windows thereby shortening engine life.
Accordingly an object of this invention is to provide a method and apparatus for converting the radient energy from the sun or other source to mechanical energy by using a reciprocating heat engine of open cycle design wherein the radiant energy is absorbed directly in the working fluid to provide higher effeciencies, lower average temperatures, using a simplier and more reliable mechanical design than hitherto available. A further object of this invention is to provide a method and apparatus to change the radiant energy from cylinder to cylinder by use of an optical valve assembly that introduces the radiant energy to the proper cylinder at the point in the thermodynamic cycle to provide the maximum possible conversion efficiency. Another object of this invention is to provide a means to absorb the radiant energy in the working fluid within the engine by a suspension of small, absorbing particles that may vaporize after fulfilling their absorption function leaving the exhaust of the engine substantially without particulate content. Further objects and advantages of our invention will become apparent from a consideration of the drawings and ensuing description thereof.